In field applications, chemical analysis instruments may be confronted with various complex mixtures regardless of indoor or outdoor environments. Such mixtures may cause instrument contamination and confusion due to the presence of molecular interferents producing signatures that are either identical to that of the chemical compounds of interest or unresolved by the analytical instrument due to its limited resolution. An interferent can also manifest its presence by affecting the limit of detection of the compound of interest. A multi-stage analysis approach may therefore be used to reduce the chemical noise and produce enough separation for deterministic detection and identification. The multi-stage analysis may include either a single separation technique such as mass spectrometry (MS) in MSn instruments or a combination of different separation techniques, such as mass spectrometry and ion mobility spectrometry. These are called orthogonal techniques since, even though they may operate in tandem, they measure different properties of the same molecule by producing multi-dimensional spectra hence increasing the probability of detection and accuracy of detection. For field instruments, such techniques may be physically and operationally integrated in order to produce complementary information hence improving overall selectivity without sacrificing speed and sensitivity.
Ion Mobility Spectrometers (IMS) using radioactive ionization have been the workhorse of trace explosives detection at passenger checkpoints in airports. The technique relies on the availability of sufficient explosives residue (particles and/or vapor) on the passenger skin, clothing, and personnel items to signal a threat. The assumption being that due to their high sticking coefficient it is difficult to avoid contamination by explosives particles during the process of handling a bomb. The same high sticking coefficient results in extremely low vapor pressures and hence makes their detection difficult. The acquisition of vapor and/or particle samples may be achieved by either swiping “suspect” surfaces of luggage or persons, or in the case of portals and/or by sending pulses of compressed air intended to liberate particles off the person's clothing, skin, shoes etc. . . . In both cases the sample is introduced into an IMS for analysis.
Ion mobility spectrometry utilizes relative low electric fields to propel ions through a drift gas chamber and separate these ions according to their drift velocity. In IMS, the ion drift velocity is proportional to the field strength and thus an ion's mobility (K) is independent of the applied field. In the IMS both analyte and background molecules are typically ionized using radioactive alpha or beta emitters and the ions are injected into a drift tube with a constant low electric field (300 V/cm or less) where they are separated on the basis of their drift velocity and hence their mobility. The mobility is governed by the ion collisions with the drift gas molecules flowing in the opposite direction. The ion-molecule collision cross section depends on the size, the shape, the charge, and the mass of the ion relative to the mass of the drift gas molecule. The resulting chromatogram is compared to a library of known patterns to identify the substance collected. Since the collision cross section depends on more than one ion characteristic, peak identification is not unique. IMS systems measure a secondary and less specific property of the target molecule—the time it takes for the ionized molecule to drift through a tube filled with a viscous gas under an electric field—and the identity of the molecule is inferred from the intensity vs time spectrum. Since different molecules may have similar drift times, IMS inherently has limited chemical specificity and therefore is vulnerable to interfering molecules.
Other mobility-based separation techniques include high-field asymmetric waveform ion mobility spectrometry (FAIMS) also known as Differential Mobility Spectrometry (DMS). FAIMS or DMS is a detection technology which can operate at atmospheric pressure to separate and detect ions, as first described in detail by I. A. Buryakov et al., International Journal of Mass Spectrometry and Ion Processes 1993, 128 (3), pp. 143-148, which is incorporated herein by reference. FAIMS separates ions by utilizing the mobility differences of ions at high and low fields. Compared to conventional ion mobility, FAIMS operates at much higher fields (˜10,000 V/cm) where ion mobilities become dependent on the applied field and are better represented by Kh, a non-constant high-field mobility term. Variations in Kh from the low-field K, and the compound-dependence of that variation aids FAIMS in its separation power. FAIMS utilizes a combination of alternating current (AC) and direct current (DC) voltages to transmit ions of interest and filter out other ions, thus improving specificity, and decreasing the chemical noise. FAIMS can reduce false positives, since two different compounds having the same low-field mobility can often be distinguished in a high-field environment.
Known atmospheric pressure ionization devices, such as the ones used in IMS and FAIMS (DMS) devices, may use a radioactive ionization source to generate the ions that are used in connection with the chemical analysis and detection processes. Still other known devices may use non-radioactive ionization techniques that include corona discharges and/or ultraviolet (UV) light and laser-induced ionization. Radioactive sources such as 63Ni are continuous sources of beta particles or high energy electrons (tens of keV). It has further been recognized that electron beams generated using non-radioactive means may be used as non-radioactive ionization sources for atmospheric pressure chemical ionization devices. In connection with the above-noted techniques, reference is made, for example, to U.S. Pat. No. 8,440,981 to Bromberg et al., entitled “Compact Pyroelectric Sealed Electron Beam,” U.S. Pat. No. 6,429,426 to Döring, entitled “Ionization Chamber with Electron Source,” and U.S. Pat. No. 5,969,349 to Budovich et al., entitled “Ion Mobility Spectrometer,” all of which are incorporated herein by reference. The use of non-radioactive ionization sources beneficially avoids certain health hazards as well as travel and legal restrictions on radioactive sources.
Accordingly, it would be desirable to provide a non-radioactive ion source for use in chemical analysis and detection devices that provides beneficial enhancements and efficiencies over known ionization devices.